This invention is concerned with techniques for processing information which is transmitted optically, such as in an optical communications system or in an optical computer.
The inherent parallelism of optics (i.e., a beam of light can carry different information on different portions of the light beam without interference) and the wide bandwidth which optical systems offer for communication are ideal for real-time image processing, optical interconnection schemes, and associative processing. As a result, optics is emerging as an area of increasing importance in high-speed information processing. Reconfigurable optical interconnections, for example, play a key role in optical computing systems, where such interconnections are used to link arrays of lasers with arrays of detectors. Conceptually, such an interconnection can be achieved by using an optical matrix-vector multiplication, where the input vector represents the signals carried by an array of lasers, the matrix represents the interconnection pattern which is to be implemented, and the output vector represents the signals which are sensed by an array of optical detectors.
When a transparency or a spatial light modulator is used as the interconnection mask in an optical interconnection, an excessively large fraction of the light entering the device is absorbed by the transparency or modulator. If the interconnect is used as a crossbar switch, for example, it exhibits an energy efficiency of only 1/N, where N is the dimension of the array. This excessive energy loss occurs in the device because a fractional portion (N-1)/N of the light energy from each element of the input vector cannot pass through the crossbar mask. Furthermore, this loss, which is sometimes referred to as the fanout energy loss, increases as the dimension N of the array increases. For a 1000.times.1000 crossbar switch, for example, as much as 99.9% of the input signal can be lost due to fanout. A loss of this magnitude is not acceptable in high speed computing applications, where signals pass through the spatial light modulator more than a billion times per second. This high processing speed would contribute to an enormous fanout energy loss in such a conventional optical interconnection system. In addition to the inherent fanout energy loss, all spatial light modulators have a finite insertion loss due to imperfect transmission properties and the scattering of light. If such insertion losses are also accounted for, the energy efficiency of a crossbar switch is reduced to t/N, where t is the transmittance (t&lt;1) of each of the optical channels through which information is transmitted in the switch.
This efficiency problem has been addressed in the prior art. It is known, for example, to employ a holographic optical element in order to achieve a free space optical interconnection. In this scheme, light from each laser source within the input array is Bragg scattered and redirected to one or more detectors in the output array. Several specific requirements, however, such as alignment, diffraction efficiency, etc., must be met for a holographic optical element to be used for the interconnection of VLSI (Very Large Scale Integration) circuits. In addition, a new hologram must be provided for each new interconnection pattern. Consequently, a need exists for a new optical interconnection scheme which can be easily reconfigured while achieving a high level of efficiency.